Hot plate welding is the oldest of the plastics welding processes. Even today, some assemblies are put together by sliding a hot piece of iron between two parts until the plastic flows and then pressing the parts together by hand. While this may be acceptable practice for some assemblies, it is hardly practical for the vast majority. Still, all of the basic principles of hot plate welding are present in this simple process. First, plastic parts are fixtured, then pressed against a hot tool until the surfaces are melted, then the hot tool is removed and the parts are pressed together until they cool sufficiently to remain joined. The key variables in the process are the heat of the plate, the pressing time, pressure, and distance traveled when against the hot plate, the speed of the change-over, that is, the time the parts are separated but not against the hot plate, and the time, pressure, and distance traveled when the parts are pressed together to effect the weld. A key feature of traditional hot plate welding is actual contact between the plastic parts and the heated tool itself. Several attempts have been made to eliminate this contact over the years, some more successful then others, but traditional hot plate welding is still a common and steady process for manufacturing a wide variety of assemblies in a dizzying array of industries.